Eucalyptus has been largely used in the pulp and paper industry in Iberian Peninsula, due to its fast growth and high productivity. This eucalyptus utilisation has generated high amounts of wastes, including leaves, branches and stumps. Hence, these wastes were selected for the co-pyrolysis studies to produce liquid fuels or raw materials. As an alternative to the conventional biomass pyrolysis, biomass was pre-treated under mild acidic conditions to obtain valuable sugar-rich stream to be used in fermentation and the solids rich in lignin were mixed with PE (polyethylene) wastes to be used in co-pyrolysis. The pre-treatment process seems to have weakened initial macromolecular structure of eucalyptus wastes and thus might have helped chemical bonds breakdown during co-pyrolysis. The results obtained so far have shown that PE presence seems to have favoured the biomass conversion. The effect of experimental conditions using Response Surface Methodology (RSM) was studied. There was a good agreement between theoretical and experimental data. The highest liquid yield (78 % wt) was obtained at 380 ºC and for the reaction time of 20 min. These conditions led to the lowest gases yield (7 % wt) and also to the lowest solids yield (14 % wt).